Grain refinement of aluminum-containing magnesium-base alloys



Patented GRAIN REFINEMIENT OF ALUMINUM-CON- TAINING MAGNESIUM-BASEALLOYS tion of Delaware No Drawing. Application October 2, 1945,

Serial No. 619,886

6 Claims. (Cl. 75-67) This invention relates to methods for refining thegrain structure of aluminum-containing magnesium-base alloys, whichcomprise treating such alloys with a suitable carbide.

It has long been recognized in the art of magnesium metallurgy that therefinement of grain structure of magnesium-base alloys was highlydesirable, inasmuch as the reduction in grain size resulted in markedlyimproved physical or mechanical properties of the magnesium-base alloycastings. Several methods were devised early in the art for refining thegrain structure of magnesium-base alloys, but most of these processesamounted merely to different methods for heat ing the molten metal priorto casting. The method which has met with the greatest success, andwhich is in general use today, comprises the superheating of the alloyabove normal melt temperatures.

Superheating of magnesium-base alloys for grain refinement may becarried out by rapidly heating the metal to a temperature in the rangeof from about 1600 F. to about 1750 F. for a period of time up to about15 minutes, after which the molten alloy is rapidly cooled. The rapidheating and cooling of the alloy in the superheating process of grainrefinement is essential, since rapid heating of the metal is required tominimize drossing and the rapid cooling of the melt is necessary toretain the benefits of superheating on the refinement of the grainstructure of the alloy. It was discovered that it was impracticable toefiect rapid heating and cooling of the molten alloy in the large potgen-,

erally employed for melting the ingot magnesium. As a result, newprocedures had to be developed inorder to satisfactorily'utilize thesuperheating methods of grain refinement.

It is thus customary, in present day processes for treatingmagnesium-base alloys, to melt the new ingots and returns in a largebulk melter of about 1700 to 2000 pounds capacity under a fluid fiuxcover. The metal is brought to a temperature considerably below thesuperheating temperature and is then poured from the bulk melter into anumber of smaller steel crucibles ranging in capacity from 60 to about500 pounds each.

Each of these smaller crucibles with its metal is placed in one of anumber of small furnaces for fiuxing and superheating treatment. Fluxing1s accomplished by stirring a layer of refining flux into the metal withan iron or steel fluxing' tool, although-in some instances the agitationnecessary for fiuxing is obtained by the use of a mechanical agitator.The fluxing operation is .the metal in small batches.

usually performed at temperatures of from about 1275 F. to 1375 F. Afterfluxing, the metal is covered again with a layer of flux, and thetemperature increased to between about 1600 F. to about 1750 F. and heldin this range for a period up to about 15 minutes, depending upon thetemperature at which the metal is superheated. At the end of the holdingtime, the crucible and metal are removed from the furnace to cool asrapidly as possible to the pouring temperature which usually liesbetween about 1250 F. to 14.75 F.

The disadvantages connected with this superheating process of grainrefinement arise grincipally from the fact that it is necessary to treatThis small batch treatment requires considerable additional equip-- mentas well as further handling of the metal. The smallness of the batchtreated and the nonuniformity of the metal from one batch to anotherintroduces factors which make large-scale production of superheatedmagnesium alloys difficult. Furthermore, the high temperatures to whichthe metal is superheated cause occasional burning of the melt as well ascontamination by deleterious gases or by certain adventitious elements,such as iron, that adversely affect corrosion and other engineeringqualities of the metal. Other disadvantages, such as critical-- ness ofthe time element and the poor working conditions resulting directly fromthe small batch treatment, which, are not of serious proportionsonlaboratory or small-scale basis, become of prime importance when theoperations assume the proportions of large-sale commercial production.

It has been suggested that carbon or carbonaceous material be used totreat molten magnesium and thus effect grain refinement. This method ofproducing grain refinement has proven highly successful, but, as in thedevelopment of most new processes, there are several disadvantages towhich no solution has heretofore been oifered. When carbon in themolecular form, such as graphite, coke, coal, etc., is employed,considerable difiiculty is encountered in obtaining dissolution of thecarbon in magnesium. These materials are very much lighter than themolten magnesium and therefore must be thoroughly puddled or mixed intothe melt. When carbonaceous material is used in which the carbon ispresent as individual atoms, that portion of the carbonaceous materialother than the carbon frequently has deleterious effects upon themagnesium melt.

7 1 It is an. object of this invention, therefore, to

v able carbide.

Another object of this invention is to provide a method of refining thegrain structure of alumihum-containing magnesium-base alloys whichcomprises treating such alloys with a carboncontaining materialwhichcontains no elements which adversely affect the magnesium melt.

Still another object of this invention is to provide a method forrefining the grain-structure of aluminum-containing magnesium-base whichmay be carried out in large melting pots at a temperature so low as toobviate any danger of adverse high-temperature efiects upon the melt.

It is also an object of this invention to provide a method of refiningthe grain structure of magnesium-base alloys which comprises treatingsuch alloys with. a solid carbonaceous material of such a character asto provide carbon in the atomic form as well as other elements which actto beneficiate the magnesium melt.

Other objects and advantages of this invention will become apparent tothose skilled in the art from the following detailed descriptionthereof.

. It has been discovered that substantial reduction in the grain size ofaluminum-containing magnesium-base alloys may be successfully achievedby treating such alloy melts with a suitable carbide. Although there aremany carbides which will effect grain refinement, this invention relatesessentially to those carbides which contain, in addition to the carbon,a metal or metals which do not adversely affect the magnesium melts.Some of the carbides which are particularly suitable in practicing thepresent invention are aluminum carbide and silicon carbide, andmanganese carbide in the form of highcarbon, high-manganeseferromanganese.

The aluminum in the aluminum carbide has an Q especial affinity for themagnesium metal and,

to a certain extent, imparts desirable physical characteristics to theresulting alloy. The manganese in the high-manganese high-carbonferromanganese material is especially desirable in that it is soluble inthe magnesium and readily dissolves, thus freeing carbon as individualatoms which are, therefore, in a form readily dissolved by the melt. Thehigh specific gravity of the ironmanganese-carbon compound also makes iteasy to introduce it as a fine powder into the melt. The manganeseabsorbed further helps to maintain the manganese above the minimumamount required in most commercial casting alloys.

In general, the method of treating magnesium melts with carbidecomprises melting the magnesium, refining the molten magnesium, andadding a carbide in the powdered form to the melt, and stirring it inthoroughly. It is immaterial whether the magnesium alloy melt is refinedbefore or after it is treated with the carbide. For example, highlysatisfactorygrain reduction has been accomplished by puddling thecarbide into the magnesium metal while it is being melted andsubsequently refining the melt that is produced. Anotherprocedure whichresults in highly satisfactory grain-refinement is to treat the moltenalloy with a suitable carbide after refinement with a flux in the usualmanner. Several methods of refining magnesium melts have been devisedand any one of these may be employed alloys along with the carbidetreatment. One method,

however, which is especially satisfactory comprises treating the moltenmagnesium with chlorine in the gaseous form according to the procedureset forth in the copending application of James C. De Haven, Serial No.619,887, filed October 2, 1945, and entitled Methods of reducingmicroporosity of magnesium-base alloy castings.

It is preferable to maintain the alloy at a temperature of approximately1400 F. while it is being treated with the carbide; however, thetreatment may becarried out from a temperature just above the meltingpoint to the temperature at which the alloy burns, depending somewhatupon the form in which the carbide is added. In certain instances whenthe carbide is added in large pieces, it is necessary to maintain ahigher temperature in order to assure dissolution of the carbide. Thecarbide may be added by sprinkling it in a powdered form over the top ofthe molten alloy. by mixing it in with the refining flux, or byenclosing it within a wire basket or a phosphorizing cup of a suitablematerial and placing the basket or cup at the bottom of the melting potwhere the carbide is generally loosened from its container. 7

Although it is not known just exactly how much carbon must be dissolvedin the alloy to eflfect grain size reduction, it has been determinedempirically that approximately 0.005% carbon dissolved in the alloy ismore than sufiicient to effect a maximum degree of grain refinement.

Wide variances in the amount of carbon added to the melt do notmaterially affect the refinement of the grain structure of the alloy,and, for this reason, from between about 0.1% to about 0.8% of thecarbide may be added in order to insure the incorporation of an adequateamount of carbon.

The following specific example is given in order to illustrate theinvention with such clarity as to enable anyone skilled in the art toreadily practice it. A quantity of a magnesium alloy consistingessentially of 6% aluminum, 3% zinc, 0.2% manganese, and the balancemagnesium was melted and heated to about 1300 F. The melt thus formedwas then cleaned and degassed by passing chlorine therethrough for aperiod of between 10 to 20 minutes in the manner described in thecopending application of James C. De Haven, above-cited. The temepraturewas then raised to approximately 1400 F., and manganese carbide in theform of high-manganese highcarbon ferromanganese ground to a mesh sizeof approximately 35 was added in a phosphorizer cup and then stirredinto the melt with a steel rod. After gently stirring in the manganesecarbide for a period of about 3 minutes, the

melt was poured at 1400 F. A /2-incli test bar test bar made of similarmetal.

The above-outlined procedure was followed with the single exception thataluminum carbide having a mesh size of approximately 35 was used inplace of the manganese carbide. The grain size of the resulting test baraveraged 0.005 inch.

Another procedure for treating magnesium alloys with carbides which isvery satisfactory, and which in certain respects is superior to theprocedure outlined immediately above, consists of puddling the grainrefining carbide into the magnesium melt as the solid charge is melted.The temperature is then raised to about 1300 F., at which temperaturethe melt would be cleaned and degassed, preferably by bubbling chlorinetherethrough as above described. The melt is then raised to the pouringtemperature of between 1350 and 1450 F. and poured into the molds.

Other variations and modifications of the present invention as abovedescribed will become apparent to those skilled in the art and it is notintended to limit the scope of the present invention except as definedin the appended claims when read in view of the specification.

It is apparent from the above description that the present inventionrelates to methods for refining the grain structure ofaluminum-containing magnesium-base alloys and comprises treating suchalloys with a suitable carbide. By the words "suitable carbide, as usedin the specification and the appended claims, is meant any carbide whichcontains no elements of such a nature as to adversely efiectaluminum-containing magnesium-base alloys. Calcium carbide, for

example, is excluded by this definition from the category of suitablecarbide, because the calcium increases the grain size and thus negativesthe effect of the carbon. By the utilization of the present inventionmagnesium-base alloys may be produced having grain sizes comparable tothose obtained by means of superheating without the accompanyingdisadvantages of high temperatures and small batch treatment. Thepresent invention further provides a method for introducing carbon intothe molten magnesium alloy in the atomic form without simultaneouslyintroducing therein elements which adversely affect the physicalcharacteristics of the alloy. f Having thus described the presentinvention, what is claimed is:

1. A method of refining the grain structure of aluminum-containingmagnesium-base alloys,

which comprises bringing such an alloy into contact with a carbide ofthe group consisting of silicon carbide, aluminum carbide, and manganesecarbide while said alloy is in the molten condition.

2. In a, method of treating aluminum-containing magnesium-base alloyswhich includes melting such an alloy, fiuxing the molten alloy, andcasting the fiuxed alloy, the step which comprises bringing the moltenalloy into intimate contact with a carbide of the group consisting ofsilicon carbide, aluminum carbide, and manganese carbide prior to thecasting thereof.

3. In a method of treating aluminum-containing magnesium-base alloyswhich includes melting such an alloy, fiuxing the molten alloy, andcasting the fiuxed alloy, the step which comprises adding between about0.1% to about 0.8% by weight of a granular carbide of the groupconsisting of silicon carbide, aluminum carbide, and magnesium carbideto such an alloy while it is heated to approximately 1400 F.

4. In a method of treating aluminum-containing magnesium-base alloyscomprising melting such an alloy, fiuxing the melt, and casting thefiuxed melt, the step which includes bringing the melt into contact witha granular carbide of the group consisting of silicon carbide, aluminumcarbide, and manganese carbide having a, particle size of about 35 +200mesh prior to casting said melt for a sufiicient period to effectdissolution of at least about 0.005% of the carbon in the metal.

5. In a method of treating aluminum-containing magnesium-base alloyscomprising melting such an alloy and fiuxing the melt, the step whichcomprises bringing the melt into intimate contact with a carbide of thegroup consisting of silicon carbide, aluminum carbide, and manganesecarbide in a divided form and agitating the molten alloy untilsufiicient carbon contained in the carbide has dissolved in the metal toeifect grain refinement of the alloy.

6. A method of refining the grain structure of aluminum-containingmagnesium-base alloys, which comprises bringing the molten alloy intocontact with a carbide of the group consisting of silicon carbide,aluminum carbide, and manganese carbide for a time sufficient for thecarbon contained in the carbide to dissolve in the metal and thus toeffect grain refinement of the alloy.

' JAMES A. DAVIS.

REFERENCES orrep The following references are of record in the file ofthis patent:

Mahoney et al., Metals Technology, vol. 12, T. P. 1892, A Study ofFactors Influencing Grain Size in Magnesium Alloys and a CarbonInocculation Method for Grain Refinement, pub. June 1945, by theAmerican Institute of Mining and Metallurgical Engineers, Inc., York,Pa., pages 16 and 17. v

Eastwood et al., Project Report Number 4. on Research Project Contract,W. P. B. 135, An Investigation of Cast Magnesium Alloys and of theExisting Foundry Techniques and Practices, published April 1, 1945, pp.222 to 229.

Certificate of Correction Patent No. 2,429,221. October 21, 1947.

JAL IES A. DAVIS It is hereby certified that errors appear in theprinted specification of the above numbered patent requiring correctionas follows: Column 2, line 12, for 14.75 F. read 1475" F.; line 35, forlarge-sale read large-scale; column 4, line 51, for temeprature readtemperature; column 6, line 8, for magnesium read manganese; and thatthe said Letters Patent should be read with these correctionsthereinthat the same may conform to the record of the case in the PatentOflice.

Signed and sealed this 16th day of December, A. D. 1947.

THOMAS F. MURPHY,

Assistant Uommissioner of Patents.

